The structure of the octameric histone core of the nucleosome has been determined by x-ray crystaflography to a resolution of 3.1 A. The histone octamer is a tripartite assembly in which a centrally located (H3-H4)2 tetramer is flanked by two H2A-H2B dimers. It has a complex outer surface; depending on the perspective, the structure appears as a wedge or as a flat disk.The disk represents the planar projection of a left-handed proteinaceous superhelix with -28 A pitch. The diameter of the particle is 65 A and the length is 60 A at its maximum and -10 A at its Ium extension; these dimensions are in agreement with those reported earlier by Klug et al.

No abstract

The structure of the (H2A-H2B-H3-H4)2 histone octamer has been determined by means of x-ray crystallographic techniques at a resolution of 3.3 angstroms. The octamer is a prolate ellipsoid 110 angstroms long and 65 to 70 angstroms in diameter, and its general shape is that of a rugby ball. The size and shape are radically different from those determined in earlier studies. The most striking feature of the histone octamer is its tripartite organization, that is, a central (H3-H4)2 tetramer flanked by two H2A-H2B dimers. The DNA helix, placed around the octamer in a path suggested by the features on the surface of the protein, appears like a spring holding the H2A-H2B dimers at either end of the (H3-H4)2 tetramer.

The common northern grapevine (Vitis labrusca L.) grows rather abundantly in the wooded area around Woods Hole, Massachusetts. It frequently grows as a liana, climbing high in various trees, and attaining heights of 17 to 18 meters. In the winter the vessels of the stem and branches are filled with gas and usually contain no sap. With the warm weather in April and MIay, and before the leaves are outs the vines become full of sap which drips out from the slightest cut in the wood of stem or twigs. The vessels are so wide and so closely packed together that one can see the light through a piece of vine 2.5 cm long, and one can easily blow air through a meterlong section. The sap will flow easily back and forth in a section when it is slightly tilted. atmospheres, or even more (6,18,20,21,27,29,30,35,38 (20,30,36,38), which sometimes gave a tension of several atmospheres. As will be shown, however, these experiments did not necessarily mean that the vessels were involved. Kramer (22) found that the transpiration rate in intact tomato and sunflower plants was more than 20 times greater than the rate at which a vacuum pump could pull sap up through the stump. Bode (6) found that, when he wilted shoots of Syringa by closing the cut end with wax, the decrease in diameter of the twig and individual vessels was greater than could be effected by a vacuum pump. Crafts (10) produced a break in the sap in the intact vessels of wilted Ribes plants by lightly tapping the vessels. The ends of the broken sap columns retracted rapidly, indicating cohesion before the break, provided no air entered. A study of the injection rate of dye solutions into the vessels of various trees led Preston (32) to estimate that cohesion of up to three atmospheres may exist in at least some vessels.Arcichovskij and his collaborators (1, 2, 3) determined the concentration of sucrose which would neither gain nor lose volume when in contact with the exposed cambium. This was done by a potometer or by observing the absence of optical streaking (Schlieren). The results reflected beautifully the diurnal transpiration cycle and showed also a correlation with height. It was assumed that the osmotic pressure of the test solution equalled the sap pressure in the xylem. A birch tree had, accordingly, sap tensions as high as almost -37 atmospheres and a desert tree gave the staggering figure of -

No abstract